The present invention relates to a current pulse circuit for the supply of a well-defined current pulse to a ferrite core memory of the type where a desired current loop is set up by means of current gates according to a coordinate system.
The current pulses which are used for writing and reading in ferrite core memories must be well defined with respect to rise time, amplitude and duration. From the effectivity point of view it is desirable that the current increases as rapidly as possible until the intended pulse amplitude is obtained after which the current is to be kept constant for a time which is determined by the time required for the remagnetization of the ferrite cores.
In the simplest case the current pulse circuit can be a resistor connected between a direct current source and the current loop being set up. Then the rise time is determined by the quotient between the resistance value and the inductance of the current loop while the pulse amplitude is determined by the quotient between the voltage value of the current source and the resistance. The disadvantage of this arrangement is that the voltage has to be relatively high in order to produce a well-defined pulse amplitude and this causes a high power loss. Besides, the linearity becomes bad since the current grows asymptotically according to an exponential function.
A better current pulse circuit is obtained by substituting an active current regulator for the above-mentioned resistor. The regulator can be controlled from a comparator supplied with a measured voltage which is proportional to the instantaneous magnitude of the current pulse as well as with a reference voltage representing the target value for the pulse amplitude. According to a known embodiment the current regulator comprises a number of parallel-connected output transistors, the number of which depends on the load, whereby the inductive current loop represents the collector load and the measured voltage is obtained across an emitter resistor. In this arrangement the rise time of the current pulse is mainly determined by the growth of the base potential towards a value which is determined by the dimensioning of the regulator, and therefore the linearity becomes very good. The condition, however, for the current regulation to work correctly is that the operating point of the collector is in the active area all the time, i.e. the collector-base voltage must be of such a magnitude that a sufficient current amplification is maintained. Consequently, the input voltage must be dimensioned with regard to the sum of the required collector-base voltage and the counter voltage being induced in the current loop by the current pulse. However, after the current pulse has reached its full amplitude the entire input voltage will load the regulator because the counter voltage has then disappeared. A further condition is that base current must not be supplied to the output transistors before the current loop has been set up, as, otherwise, the base region would be saturated with charge carriers and, consequently, the regulation would be eliminated. Therefore the current regulator has to be activated after the desired current loop has been set up and be deactivated before the current loop is disconnected. Thus the disadvantages of the known arrangement are (1) that it is expensive because it requires a relatively high input voltage, which results in a high power loss in the current regulator, and that (2) it has to be activated after the desired current loop has been set up, which results in a certain time delay and consequently restrains the reading and writing speed of the memory.